It is well-known that a drawback of silver is the tendency to form a dark coating (tarnish) on its surface under normal atmospheric conditions. The tarnish forms quickly and is difficult to remove.
Prior art cleaning compositions for silver items are conventionally of two types, namely (a) abrasive polishes and (b) chemical solvents for the tarnish. As well as these, electrochemical methods have been used. Polishes abrade the tarnish and may also wear away the silver itself, which is a problem particularly with EPNS, and the fine abrasive materials used can lodge in intricate decorations on the surface of the object, from where they are hard to remove.
Currently, the most common form of chemical solvent is a "dip cleaner"; these are acidic solutions the main active ingredient of which is thiourea (NH.sub.2 CSNH.sub.2).
Thiourea and its derivatives have been known for many years for use in cleaning metal surfaces. They are best known as inhibitors in the acid cleaning treatment of ferrous metals. For example, U.S. Pat. No. 1,608,622 (Schmidt et al) of Nov. 30, 1926 discloses the use of thiourea admixed with sulphuric acid for the pickling of iron and steel.
U.S. Pat. No. 2,071,966 (Bolton) of Feb. 23, 1937 discloses the use of substituted thioureas containing a hydrogenated aryl group admixed with an acid for cleaning metal surfaces.
GB 1 242 920 (Abbott Laboratories) of Aug. 18, 1971 discloses the use of thiourea derivatives for use as corrosion inhibitors for ferrous and aluminium metal bodies, particularly for use in oilfield applications. One technique suggested for the application of the inhibitor to production well casings is to form a "stick" comprising inhibitor and binders; such sticks are then dropped into the well tubing. After falling to the bottom of the well, the stick disintegrates or melts to release the inhibitor. Preferred binders for producing these inhibitor sticks are lead oxide, petroleum wax and asphalt.
U.S. Pat. No. 2,628,199 (Lowenheim) of Feb. 10, 1953 discloses a cleaning composition for silver- and copper-containing metal surfaces which comprises about 3 to 5 parts by weight of thiourea together with 1 part by weight of a normally solid acid having a pK in the range 1 to 5; preferred acids disclosed as therein are citric, tartaric and oxalic acids.
The more modern dip-cleaners are liquid formulations into which a tarnished silver article is immersed or dipped in order to remove the tarnish, leaving a clean metal surface. These dip cleaners suffer from a number of serious disadvantages in that they are hazardous to keep around the home, cause damage to surfaces if spilt, tend to precipitate over time, and develop an unsightly build up of tarnish in the product after use. Furthermore, there is no means of telling when the active ingredient is spent.
The present invention therefore seeks to provide a silver cleaner in a form that will remove tarnish without damaging the surface and which will not leave deposits in surface features and decorations, and which furthermore avoids or at least ameliorates the handling and storage problems mentioned above.